Monday, June 25, 2012

Célia Fonseca Guerra at the Vrije Universiteit Amsterdam has made the exercises she uses in teaching a bachelor level course on computational chemistry available here. The exercises are based on the ADF program, but could be adapted to other programs without too much trouble.

Tuesday, June 5, 2012

The complete basis set (CBS) limit is not a basis set though it is often written as such, e.g. B3LYP/CBS. Instead the CBS limit is an extrapolated estimate of a result obtained using an infinitely large (complete) basis set. In principle this procedure removes any error due to the linear combination of atomic orbitals approximation, and any remaining disagreement with experiment is due to some other approximation such as the treatment of correlation. For many properties the CCSD(T)/CBS value can be regarded as a numerically exact for all practical purposes, i.e. it is unlikely that any higher level of theory predict significantly better results.

The extrapolation is based on a minimum of three separate calculations with increasingly larger basis sets. CBS limit extrapolation works only with basis sets designed specifically for the task, such as the correlation- or polarization-consistent basis sets, e.g. cc-pVxZ or pc-n.

The procedure is as follows: a given property $Y$ of interest (e.g. a relative energy, a frequency, or a bond length) is computed at a given level of theory (e.g. B3LYP) using at least three basis sets (e.g. cc-VDZ, cc-VTZ, and cc-VQZ. These data points a then fit to an equation, the two most popular equations are given here

$Y(x)=Y_{CBS}+Ae^{-Bx}$ (1)

$Y(x)=Y_{CBS}+Ax^{-3}$ (2)

Here, $Y_{CBS}$ is the CBS limit we're after and $x$ is 2 for cc-pVDZ, 3 for cc-pVTZ, and so on. $x$ is also often written as $L_{max}$ (or $l_{max}$), which is the highest angular momentum included in the basis set. For cc-pVDZ this means $d$ orbitals, which have an angular momentum of 2, so $x$ and $L_{max}$ are really the same.

Equation (1) contains three parameters ($Y_{CBS}$, $A$, and $B$) so a minimum of three different basis sets are needed to determine them. While Equation (2) only has two parameters, a minimum of three data points are still needed for reliable results.

Friday, June 1, 2012

CCH is an overlay journal that identifies the most important papers in computational and theoretical chemistry published in the last 1-2 years. CCH is not affiliated with any publisher: it is a free resource run by scientists for scientists. You can read more about it here.

Table of content for this issue features contributions from CCH editors Steven Bachrach, Dean Tantillo, Dmitri Fedorov, and Jan Jensen: